1. Field of the Invention
The present invention relates a digital hearing aid or hearing aid for perceptive deafness employing a digital signal processing.
2. Description of the Related Art
A hearing impairment, i.e. deafness, is generally classified into two kinds, i.e. conductive deafness and perceptive deafness. The conductive deafness is a hearing impairment caused for variation of transmission characteristics due to failure of any one or all of auris externa, auris media, fenestra cochleae, and fenestra ovalis. This type of hearing impairment can be simply overcome by amplifying input sound. On the other hand, the perceptive deafness is a hearing impairment which is considered to be caused by organic failure in a certain portion from auris interna to cortical auditory area, and represents a condition causing difficulty in perceiving sound due to abnormality of auris interna or so forth. Such difficulty of perceiving sound can be caused by dropout of stereocilium at the tip end of hair cell of the cochlea or by failure of a nerve transmitting voice. Also, senile deafness is involved in this type of deafness. The perceptive deafness is difficult to overcome by the conventional hearing aids which simply amplify sounds. In recent years, attention has been attracted to a digital hearing aid which can perform complicated signal processing.
There is a significant difference of symptom of perceptive deafness in each individual. One primary symptom of perceptive deafness is recruitment of loudness. This is the phenomenon to raise a minimum level (minimum audible threshold: HTL) and to maintain a maximum level (maximum audible threshold: UCL) as substantially unchanged, to thereby narrow an audible range (audible area), as shown in FIG. 6. Also, the maximum audible threshold is frequently lowered slightly. Namely, this is the phenomenon to cause difficulty in hearing a low level sound but to hear a high level sound in substantially equal level to a person having normal hearing ability. If the sound is audible by the hearing aid for making the low level sound audible, the output sound of the hearing aid upon inputting of high level sound should exceed the maximum audible threshold, to be a discomfortable level to perceive. For this reason, it becomes necessary to amplify low level sound with a high amplification, and to amplify high level sound with a low amplification. It is also one characteristic of perceptive deafness in variation of the hearing acuity per frequency level.
The first prior art taking a measure of the perceptive deafness will be discussed hereinafter. The first prior art has been proposed in Japanese Unexamined Patent Publication No. Heisei 3-284000. In the disclosed technology, a dynamic range of an input sound is compressed into an audible range of the deafness. FIGS. 7(a) to 7(e) show an acoustic sense compensation method of a hearing aid employing a method disclosed in the above-identified publication. FIG. 7(a) is a graph taking an acoustic pressure on the horizontal axis and a loudness on the vertical axis. Acoustic pressure is a physical amount of sound and loudness is a magnitude to be felt by a listener as hearing a sound of certain acoustic pressure, namely sensory amount. In the graph, a solid line represents a relationship between the acoustic pressure and the loudness as heard by a person having healthy or normal acoustic sense, and a broken line represents a relationship between the acoustic pressure and the loudness as heard by a person having deafness. As can be appreciated from FIG. 7(a), a sound having a given level of acoustic pressure is heard by people one having healthy acoustic sense and the other having deafness, the person having healthy acoustic sense feels greater magnitude of sound than the person having deafness. When the acoustic pressure to be heard becomes lower than the minimum audible threshold, while the person having healthy acoustic sense can hear the sound, the person having deafness cannot hear. FIG. 7(b) shows the acoustic pressure feeling equal loudness level in the person having healthy acoustic sense and the person having deafness. In FIG. 7(b), the vertical axis and the horizontal axis respectively represent acoustic pressure level for the person having deafness and acoustic pressure level for the person having healthy acoustic sense. Difference of the sound to be felt at equal level by the person having deafness and the person having healthy acoustic sense increases according to decreasing of the acoustic pressure and decreases according to increasing of the acoustic pressure. In FIG. 7(b), the broken line represents the result of comparison of the acoustic pressure level to be heard at equal loudness level between people having healthy acoustic sense. As can be seen, in this case, increasing of the acoustic pressure becomes linear. In FIG. 7(b), considering that the acoustic pressure level for the person having healthy acoustic sense is input and the acoustic pressure level for the person having deafness is output, by amplifying an input sound by the hearing aid with taking a difference between the broken line and the sloped line in FIG. 7(c) as an amplification, the person having deafness may feel the equal magnitude of the sound as that felt by the person having healthy acoustic sense. FIG. 7(d) shows a relationship between amplification to be derived as set forth above, and an input acoustic pressure. As can be seen, when the input acoustic pressure is lower, the amplification becomes greater, and when the input acoustic pressure is higher, the amplification becomes smaller. FIG. 7(e) is a conceptual illustration of a method for deriving an amplification of the hearing aid on the basis of the loudness curves of the person having healthy acoustic sense and the person having deafness and magnitude of input sound. In FIG. 7(e), the vertical axis represents the loudness level (phon) and the horizontal axis represents the acoustic pressure level (dB) of the input sound. The solid line is a loudness curve of the person having healthy acoustic sense and one-dotted line is a loudness curve of the person having deafness (hereinafter occasionally referred to as "user of hearing aid" or simply as "user"). FIG. 7(e) illustrates the magnitude of sound to be heard by the person having healthy acoustic sense and the user of the hearing aid. For example, the sound heard at a level c' by the person having healthy acoustic sense has the acoustic pressure of c, whereas the sound heard at the level c' by the person having deafness has the acoustic pressure of c". Namely, when the sound having the acoustic pressure of c is amplified to have the acoustic pressure of c" to make the person having deafness to hear, the person having deafness may hear the sound in substantially equal level as that heard by the person having healthy acoustic sense. That is, the amplification of the hearing application is that necessary for amplifying the acoustic pressure of c to the acoustic pressure c". In FIG. 7(e), both of the vertical axis and the horizontal axis represent logarithmic values. Therefore, the amplification can be derived from the following equation. EQU G=c"-c
wherein G is an amplification, c" is the magnitude of sound to be heard by the person having deafness and c is the magnitude of the input sound.
As can be appreciated from the foregoing equation, the amplification becomes greater at greater difference of c" and c.
On the other hand, even when the foregoing measure for the perceptive deafness is taken, narrow audible area of the person having deafness in comparison with the person having healthy acoustic sense is held unchanged, and magnitude of all sound to be heard is unified. Particularly, difficulty is caused in feeling large sound and small sound. As a result, in the output signal provided dynamic range compression process as set forth above, it is difficult to distinguish the input sound which the person having healthy acoustic sense feels significant level, such as horn of an automotive vehicle, a bell sound of a fire alarm box, a siren of police patrol car or the like and so forth, from other input sound. As a measure for such problem, the second prior art has been proposed in Japanese Unexamined Patent Publication No. Heisei 4-148396. In the proposed technology in the above-identified publication, the bell sound of the fire alarm box is noticed to the person having hearing impairment by vibration with employing an oscillator operative in response to actuation of a smoke detector, and a portable vibrator which can be driven and stopped by an output of the oscillator. FIG. 8 shows a general illustration of the system established for this purpose. As shown in FIG. 8, the person having hearing impairment holds the vibrator driven in response to actuation of the smoke detector. When the smoke detector detects occurrence of fire, the oscillator is actuated to drive the vibrator held by the person having hearing impairment. In response to the signal from the oscillator, the vibrator notifies occurrence of fire to the person having hearing impairment in place of the fire alarm box.
In the case of the first prior art, when the acoustic pressure level is low, the input signal is amplified with large amplification and when the acoustic pressure level is high, the input signal is amplified with small amplification. As a result, variation of the input signal becomes smaller to cause difficulty in distinguishing the input signal in a specific frequency band from other environmental input sounds.
On the other hand, in the case of the second prior art, the person having hearing impairment may not detect the alarm when the oscillator generating the signal for the oscillator is not present.